Part of the increase comes from new users signing up for the apps. (That might explain why there's a spike in session length for the first couple of days in January, as they experiment with apps for the first time.) But existing app users increased usage 32 percent from December to January.

And usage—time spent, sessions per day, and length of sessions—appeared to be sustained through January.

Of course, people who download fitness apps might be more committed and inclined to keep their resolutions than the general population. Or the apps might help them stick with their resolutions. Since Kiip only studied app users, its study can't address that.